No Middle Ground offers a novel and intriguing theory to explain the polarization in contemporary legislatures: informal party organizations (interest groups, activists, and various political insiders) drive elite polarization via their control over primary elections. Because candidates need the support of activists and party insiders to win primary elections, they adopt extreme positions to appease these individuals. The sources of elite polarization are outside the chamber, rather than inside it.

The book analyzes California politics and uses that case to draw broader conclusions about parties and polarization more generally. The bulk of the empirical analysis proceeds in two parts. First, Masket analyzes 150 years of roll call voting records (going back to 1851), and explores how over-time variation in institutions (and informal party organizations) corresponds to changes in polarization. This is a monstrous data collection effort, and this will undoubtedly become an important dataset for legislative and state politics scholars in the years to come.

Second, Masket cleverly exploits a natural experiment that exogenously varies the influence of outside activists. In the middle decades of the twentieth century, California experimented with “cross-filing,” a process through which candidates could run in the primaries of multiple parties without specifying the one to which they belonged. Candidates could therefore seek the nomination of both parties, without needing to hew to the wishes of the local party organization, given that the party apparatus and activists were no longer crucial to winning a primary election. As a result, legislative parties waned as an influence on members’ behavior. So without outside activists to pressure members, the legislature depolarized. But once cross-filing was eliminated in the early 1950s, the legislature began to polarize again, as parties—and ideologically committed activists—were once again crucial to winning a partisan primary. It was this fear of outside activists that drove the parties apart inside the legislature. In short, extra-legislative actors drive legislative polarization.

While the polarization connection is one important contribution, Masket’s development of the concept of an informal party organization is equally important. Parties are no longer patronage-driven machines, but are now a loose confederation of ideologically similar activists, interest groups, and political insiders (hence the informal modifier). Ideology has replaced patronage as the glue that binds parties together (pp. 192–3).

This view challenges the existing view that parties are simply the creatures of politicians (Aldrich 1995Go), and demonstrates that the politicians themselves are also creatures of parties. Parties trade resources like endorsements, expertise, manpower, and money for fealty in the legislative arena. Much of the evidence for this expanded view of parties rests on elite interviews with leaders from five informal party organizations throughout the state (Orange County Republicans, South Los Angeles Democrats, Eastern Los Angeles County Democrats, West Los Angeles Democrats, and Fresno Democrats and Republicans) and historical archival work. The evidence here is considerably more speculative (as the author admits). In light of the considerable difficulties in gathering this sort of evidence, however, even the basic qualitative and quantitative data he does manage to present is impressive. This model of parties (closely linked to the one developed by Cohen et al. 2008Go) is as an important addition to the literature. Indeed, I suspect this book will prod scholars to consider the consequences of this new type of party organization for debates over party in the legislature and party in the electorate.

As is true of most good books, this one raises as many questions for future work as it answers. Here, I want to highlight two of them in particular. First, take the question of how informal party organizations might be responsible for polarization in the nation at large. Masket argues that the contemporary resurgence of polarization is due in large part to a resurgence of local party organizations (pp. 3, 8). This may well be true, but it raises two related points. First, why did local parties re-emerge in the second half of the twentieth century? What caused them to return and become more vigorous? Is this simply a consequence of the explosion in interest groups in the 1960s (Berry 1984Go), or is there some other factor? Second, the argument that polarization more generally is caused by the growth of informal party organizations is a hypothesis, not a conclusion. Both of these points need to be developed more fully before it can be demonstrated that informal party organizations are a key driver of polarization throughout the nation.

Second, Masket argues that informal party organizations drive polarization via their control of primary elections. While this argument is intuitively appealing—and Masket’s evidence from California is compelling and persuasive—other work demonstrates that primaries are only a minor cause of elite polarization (McCarty, Poole, and Rosenthal 2006Go; Hirano et al. 2008Go). The reader is left feeling torn between these two seemingly strong arguments concerning the relationship between primaries and polarization, with no real way to resolve them. I suspect that Masket’s work will inspire others to offer a more detailed account of how primaries generate polarization that overcomes previous null findings, but I do not think we are quite there yet.

Yet in the end, these are minor quibbles, especially given that Masket does not set out to solve the elite polarization puzzle for the nation as a whole. This book is a welcome addition to the American politics literature. Scholars from a variety of subfields—most notably polarization, legislative politics, political parties, and state politics—will want to add this book to their reading lists.